Pressurized-water nuclear reactors comprise a core consisting of prism-shaped assemblies arranged side by side in a vertical position. The assemblies comprise a framework which is closed by means of the connectors and in which the fuel rods are arranged.
One of the connectors of the assemblies, called the lower connector or "bottom nozzle", comes to rest on the lower core plate which is pierced with holes in the region of each of the assemblies to allow the cooling water of the reactor to pass through the core, in the vertical direction and from the bottom upwards.
The cooling fluid for the fuel rods passes through the adaptor plate of the lower connector via orifices, called water passages, which are either of circular (of a diameter of approximately 7 to 10 mm) or oblong (an aperture approximately 10 mm wide by 15 to 50 mm long). Debris which may be present in the primary circuit of the reactor is liable to be carried along by the circulating pressurized water, and if it is of small size (for example, smaller than 10 mm) this debris can pass through the adaptor plate of the lower connector, the water passages of which have a large cross-section. This debris can become jammed between the fuel rods and the cells of the first grid, i.e., the spacer grid, holding the rods in the form of a regular network, arranged as low as possible in the assembly. This debris, subjected to the axial and transverse hydraulic stresses, which are appreciable in this zone, can wear produce on the jacket of the fuel rod. This may result in a loss of sealing of this jacket and an increase in the rate of activity of the primary circuit of the reactor.
Devices making it possible to filter the cooling fluid of the reactor, either during the hot-running tests or even during the operation of the reactor, have therefore been proposed.
In the first case, the filter elements can be connected to the lower core plate and arranged thereon in the position of the fuel assemblies before the fueling of the core, as described, for example, in FR-A-2,577,345.
In the second case, the filter elements are associated with the fuel assemblies and are generally arranged in their lower connector. The filter elements fastened in the lower connectors of the fuel assemblies usually consist of sheet-metal or metal-wire structures which make it possible to stop the debris whose size is smaller than the largest dimension of the passage cross-section between a fuel rod and a grid cell.
Such filter elements are described, for example, in the documents US-A-4,664,880, US-A-4,684,496 and EP-A-0,196,611.
Such devices can be complex and introduce a relatively high head loss into the circulation of the cooling fluid through the fuel assembly.
Furthermore, these devices placed in the lower connector of the assembly can be relatively bulky and obstructive during the operations of loading and unloading of core assemblies and during the dismantling and refitting of the connections of the guide tubes and of the lower connector.